I thought this might be doable with a hand brace, but I'm having difficulty sourcing an irwin/jennings bit of that size. I've found large flutes called "ship augers", but they are designed for large power drills (based on the chuck shape required to fit them). In the auger kits that fit square-tapered bit braces, the largest auger in the kit is often smaller, at 1" or 1-1/2".

What type of brace bit should be used for larger holes? Is boring large holes into a thick slab, with a hand brace, a bad idea?

How deep do you need the hole? For larger holes, I cut a hole in scrap plywood using an adjustable circle cutter on the drill press. Then I clamp the plywood over the work-piece and carve the hole with a router and a template bit (one with a bearing above the cutter).
– Stephen MeschkeMay 31 at 16:31

It's to bore through a benchtop slab, so 3-4". I like the router idea. The legs on the plan are meant to be splayed (not straight down), so the template would need to be held at an angle with some wedge however.
– ww_init_jsMay 31 at 19:21

Just to add something to the Answer (which in more recent times is the way holes like this were done) another option is the much older centre bit. These are available up to at least 2".
– GraphusJun 1 at 5:21

What material is the bench top made out of? If it’s a hardwood you may have a hard time with an adjustable auger bit unless you can find one of the heavier duty ones that were made and with a fine pitch lead screw. If you find one of those or its softwood an adjustable auger would work great. Also the center bit is a good idea. They work really well but like an adjustable auger they are harder to bore a straight hole since there is nothing to guide the bit like a longer auger but.
– T. M.Jul 5 at 23:15

2 Answers
2

You may find that you need a brace with a larger than normal sweep for boring a hole this size. They were manufactured in various sizes. A more common configuration for an auger of this diameter would be something like a barrel eye pattern. They are still available new - here

The advantage of these is the additional leverage given from the handle, and they're generally tough enough to drill through oak.

The alternative is a flat bit - simple and cheap. Make up a block such that it has an angled face so the bit starts horizontal, and then clamp the block above your desired hole to ensure your bit will follow the correct direction. Augers or flat bits are not good at starting at an angle, and need help.

There are also modern saw tooth versions of forester bits which will do the job, using power tools, but again, for a typical splayed leg angle, they may need some alignment help, despite their accurate habits.

A

Further information
An adjustable bit is weaker than a solid one. You will find it very difficult to start it at an angle, as it makes contact at only one point of It's sweep. Further, if you are using beech or similar for your bench top, then I'd be very careful. The lead screw is fat, and the cutter is weak. A recipe for a broken bit. A oak-beamed tiber ark is still in existence with a part of one of my adjustable bits. Of course, that's just one instance, but my spider senses were screaming 'bad idea' and they're usually to be relied on. The urgency of completing the restoration, on site, in the woods, cost me the bit, and a return visit.

You can also make your own scraper version to enlarge a hole, or you could go with square mortices - Whatever Chris says in his book. Back in whenever one would have access to a lathe of some sort for the legs, and they would be made to whatever large size auger was on hand. Spoon bits on twenty foot irons existed for boring out water pump timbers and for string on line. Needs a skilled hand to use them of course.

Any noteworthy difference in the difficulty of cutting with those vs a double lipped fixed-size one? (aside from the obvious single cutter and single spur).
– ww_init_jsMay 31 at 20:24

Yup, this! They're slower, can/do take more pressure against the wood but they're very effective...... probably why they continued in production for more than a century :-) There's bound to be a YT video or two showing them in use if you want to see one in action rather than going by photos.
– GraphusJun 1 at 5:12